This invention relates to aqueous polyurethane dispersions and methods of manufacturing them.
Aqueous dispersions of polyurethanes are well known. They are used, for example, in making various types of films and coatings.
For several reasons, it is sometimes desirable to incorporate other polymers of various types into a polyurethane dispersion. This can be done, for example, by blending a dispersion of polyurethane particles with a dispersion of particles of some other polymer. An example of this technique is described in U.S. Pat. No. 5,547,710 to Satgurunathan et al.
U.S. Pat. No. 5,314,942 to Coogan et al. describes a process where a polyurethane dispersion is made and particles of a second polymer are prepared in situ in a separate polymerization step. Similar processes are described in U.S. Pat. No. 5,521,246 to Tien et al., U.S. Pat. No. 5,786,420 to Grandhee and WO 93/24551. These techniques require multiple polymerization steps in order to from high solids dispersions. In addition, most of these processes include a monomer feed step that requires the fed monomer to diffuse through the aqueous phase to the particles of chain-extended polyurethane. The kinetics and particle nucleation mechanisms are difficult to control in these processes due to a significant amount of polymerization that occurs within the aqueous phase. This leads to coagulation and increased viscosity. Moreover, although some of the monomers may migrate to the polyurethane particles, a significant amount of the monomers nucleate in the aqueous phase to form new particles, which do not contain polyurethane and thus do not have the desired morphology.
The process described in U.S. Pat. No. 5,451,653 to Chen et al. purports to make a hybrid dispersion by simultaneously polymerizing a monomer and chain extending a prepolymer. A special type of activated keto methylene compound is used as a crosslinker. As is typical of many processes for making hybrid dispersions, the prepolymer contains ionic groups (typically carboxylate groups) in order to impart stability to the dispersion. This makes the stability of the dispersion sensitive to pH.
It would be desirable to provide a simple, inexpensive process by which dispersions of hybrid polyurethane particles can be prepared in a single step in which the composition of the particles and their morphology can controlled